March 29, 2018
The Feast of the Nativity (i.e. Christmas Day) always occurs on December 25, why does the Feast of the Resurrection (i.e. Easter Day) move around?
The answer to this question is complicated. The bulk of the answer can be attributed to the difference between lunar and solar calendars. There are also other factors affecting the date of Easter, including eschatological expectations (i.e. when the present age would end) of the early Church, possibly, some anti-Jewish sentiments of the Nicene Council, and the inaccuracy of our time measurements for the lunar and solar year during the period of the first universal councils.
The lunar calendar versus the solar calendar is easy to explain, but it still involves human factors as well. By the time of Jesus, Rome was on the Julian, or solar, calendar. But Jesus and his followers were on the Jewish Calendar, which is lunar. It is clear from the Gospels, that Jesus' last supper and crucifixion occurred around the Jewish festival of the Passover, 15 Nissan on that calendar, usually April. Religious people being conservative about change (Episcopal Clergy still dress in garments of the Roman Empire for the celebrating of the Eucharist today), we can probably understand the commitment of the early church to come as close to celebrating Easter on, or around the Feast of Passover as they could.
The early followers of Jesus were expecting his immanent return and did not record the date of the resurrection. As time went on and a delayed Parousia became the norm, tradition developed to have the celebration related to the date of Passover. But this decision was not applied in all churches across the Roman Empire, and when Constantine called for the Council of Nicaea in 325, one of the eight areas to be covered was the dating of Easter. It was this council that agreed, in a political compromise, that Easter would not occur on the first day of Passover (this is the possibly anti-Jewish sentiment mentioned above), but that it would occur on the first Sunday after the first full moon after the Vernal Equinox. Again, not only was the dating of Passover important in the dating of Easter, but also the strong Biblical traditions (see Matthew 28:1; Mark 16:2, 9; Luke 24: 1, John 20:1, 19; Acts 20:7, and 1 Corinthians 16:2) of the resurrection occurring on the first day of the Jewish Week (i.e. Sunday).
Finally, we arrive at the accuracy issue. The lunar year has 12 cycles of 29.53 days or 354.36 days per year. The Julian Calendar (i.e. the old solar year) had 365.25days per year. Doing the math means that the lunar year is almost 12 days shorter than a lunar year. So, at the Council of Nicaea a formula was developed to make the adjustment. This adjustment was further compounded when the measurement of the solar year was determined to actually be 365.242 days, which over a century does not make much of a difference, but after 1000 years of Christian dating of Easter had thrown the date of Easter off of the solar calendar by almost 10 days. Hence Pope Gregory decreed an adjustment in the 13th century to get us back on line.
See, complicated, or at least multi-faceted. And when in question, see the BCP pages 880 through 885.